1. Field of the Invention
The present invention relates to a torque control system for an engine which is connected to a V-belt type continuously variable transmission.
2. Description of the Prior Art
Conventionally, as a continuously variable transmission for a vehicle, there is, for example, a continuously variable transmission with a V-belt. Generally, in the continuously variable transmission, a gear ratio of the vehicle such as an automobile is continuously controlled by supplying a hydraulic pressure (hereinafter referred to as a primary pulley pressure) which is obtained by regulating a line pressure as an original pressure with a shift control valve to a cylinder chamber of a primary pulley, increasing and reducing the primary pulley pressure with the shift control valve to change a groove width of the primary pulley and changing a radius ratio between the primary pulley and a secondary pulley. This shift control valve is connected to the vicinity of a middle of a shift link constituting a mechanical feedback mechanism, and a movable flange of the primary pulley and a step motor that is a shift actuator are respectively connected to both ends of the shift link. In order to prevent generation of disagreement between an operating position of the step motor for stroking the shift control valve to a position corresponding to a desired shift command value and a command value to the step motor, an initialization operation of the step motor is performed when the vehicle is stopped or an engine is started.
An example of the initialization operation of the step motor in this conventional system will be described. When an ignition switch is turned on, a motor initializing means drives the step motor to a hardware limit position in one direction and thereafter returns the motor to a reference position in the other direction, and after the motor initialing means completes the above initialization operation of the step motor, a command value to the step motor is initialized to a value corresponding to the reference position. This causes the command value to accord with the operating position of the step motor with a high precision, whereby shift control is performed accurately. (see Patent Document 1: Japanese Patent Application Laid-Open Publication No. 8-178063)
Moreover, since it is necessary to actuate the step motor from a current motor operating position to the hardware limit position in the one direction when the initialization operation of the step motor is performed, the continuously variable transmission has an EEPROM capable of storing the current motor operating position of the step motor and pulley ratio as pulley information. The EEPROM is provided with a backup power source for writing data and receives electric power supply from the backup power source to store the pulley information even if a power source of the vehicle is turned off.
However, it is impossible to write the pulley information in the EEPROM when trouble of the backup power source disenables electric power supply. Since the current motor operating position is therefore unclear, when the motor is driven to the hardware limit position of the one direction in the initialization operation of the step motor, the present operating position of the motor is presumed to be at a farthest position from the hardware limit position, and the step motor is driven for a time equal to time required until the operating position of the step motor moves from the presumed position to the hardware limit position. This causes the increase of time for performing the initialization operation of the step motor.
Since the step motor is thus operated to a direction in which the hardware limit position of the one direction, that is, the pulley ratio reaches a lowest value Lo during the initialization operation of the step motor, the primary pulley pressure is reduced because of communication with a drain side. If the driver deeply depresses the accelerator at that time, the short of the primary pulley pressure causes slippage of a belt, and the durability of the belt is degraded.